In recent years, various systems and devices have been employed in automotive vehicles to provide a warning signal whenever any occupant of the vehicle has not fastened his or her safety belt prior to operation of the vehicle. Such systems have incorporated mechanical switches under the fundament-supporting portions of the vehicle seats to detect the presence of an occupant, or in association with the seat belt so that the switch is actuated upon pulling one portion of the seat belt out of its enclosure (reel switch) or upon latching the seat belt buckle (buckle switch) to detect seat belt usage. Mechanically-variable capacitances have also been used as the means for detecting seat occupancy.
In practice, it has been found that mechanical devices for sensing seat occupancy have produced a warning signal when none should be given because these devices are all pressure-operated. Thus, when a package or pet animal is resting on the seat adjacent the driver, the system responds to the weight exerted by such non-human seat occupant and generates a signal to indicate that the seat belt buckle should be fastened. This problem has been overcome by employing capacitance-responsive systems and antenna arrays with discrimination capability as disclosed and claimed in the cross-referenced applications. However, there remains the problem of reducing the number and complexity of the signal processing circuitry for handling the occupancy and seat belt buckle condition signals from each seat. Toward this end, applicant has designed a system employing only a single seat occupancy detection circuit and which processes the signals of both first and second classes (i.e., signals indicating seat occupancy and seat belt usage, respectively) from each of four seat locations. The system disclosed herein affords the further advantage of essentially zero standby power consumption, which is an important feature in automotive applications.